Shawn Coburn farms land that holds senior water rights to the giant Central Valley Project, rights that usually assure him water.

Not this year. He already has decided to let his pomegranates die, abandon alfalfa and cut his tomato crop by half. He may not plant any row crops if the state water board follows through on its intention to slash deliveries to "protect human health and safety" from the effects of drought.

Coburn, 45, says his ranch near Dos Palos (Merced County) is no water-guzzler. He uses buried irrigation. Computers tell him how much moisture his plants lose each day.

"I need every drop of water to keep the trees and vines alive," he said. "I can't conserve any more. This year I'm going to watch stuff die."

As California gets drier and hotter, no one is more vulnerable than farmers. And no one is likely to have to do more to adapt to what many experts fear will be a more drought-prone environment.

Wenger said this year's drought has farmers asking how long they can continue. "It's going to be difficult," he said. "We're going to see a lot of farmland retired."

Straining the system

During more reliably rainy decades, California replumbed its rivers to transform the state into an agricultural Eden, the richest farm region on the planet. Water that flowed from the Sierra to San Francisco Bay was diverted to the megalopolis of Los Angeles, to what was to become Silicon Valley, and to the farms of the San Joaquin Valley.

Designed in the middle of the last century, that system is now straining at the seams. Federal authorities recently joined the state in announcing that they were unlikely to deliver any water to farms this year, which would be the first time that has ever happened.

Although recent storms have eased the shortfall, rain totals this year remain among the worst on record. It is the third serious drought since the 1990s.

Can California grow from 38 million people to a predicted 50 million in three decades and still provide half the nation's fresh produce? Experts say yes. Even ardent environmentalists said California farming is too productive, too rich and too vital just to disappear.

"No one I know of talks about stopping agriculture in California," said Peter Gleick, co-founder of the Pacific Institute, an Oakland sustainability think tank. "California is a great place to grow food. The climate's great, the soils are great, and there's plenty of water. There may not be enough for everyone to do everything we want, but there will always be enough for a strong agricultural economy.

"It just needs to be a 21st century agricultural economy," Gleick said, "not a 19th century one."

City dwellers will have to conserve as well, and the lush lawns of suburbs could someday be a historical anachronism. Future presidents may be unable, as President Obama did last month, to talk drought in Fresno one day and golf on green fairways in Rancho Mirage (Riverside County) the next.

But most of the savings will come from farming, if for no other reason than it uses 80 percent of the state's water.

The challenge for California agriculture in coming decades is likely to be stark. Juliet Christian-Smith, California climate scientist for the left-leaning Union of Concerned Scientists, said her studies of the effect of climate change on the state's farms show "pretty severe consequences past mid-century."

The Sierra snowpack, which provides more than two-thirds of the state's water, is expected to drop to 40 percent of its current historical average by the end of the 21st century, said Jeanine Jones, drought manager for the state Department of Water Resources.

Dams not the answer

"We will have lost a key reservoir that now sits up in the mountains and meters snowmelt runoff to us," Jones said. "Our existing infrastructure, much of which was designed back in the 1940s, 1950s and 1960s, wasn't designed for this hydrologic condition."

Farmers tend to see more and bigger dams as a solution, but many experts believe California can no longer plumb its way out.

"You can build all the storage you want right now, but there's no water to fill it," said Jay Lund, director of watershed science at UC Davis. "It's not like a pile of concrete in a river is going to create water."

Faced with water cutbacks imposed on them in recent years by environmental legislation and litigation, farmers adopted sophisticated irrigation systems. Central Valley farmers now use 2 million acre-feet less water than 25 years ago and grow twice as much per gallon, said Timothy Quinn, chief of the Association of California Water Agencies. An acre-foot is what would cover a football field with a foot of water and supply two California households for a year.

Yet in achieving those savings, other problems emerged. Drip irrigation saves water but also can increase groundwater pumping, which exhausts aquifers and in some places actually causes the land to sink. Drip irrigation also intensifies salt buildup in soils that damages crops.

Environmentalists argue that some crops shouldn't be grown at all. They accuse farmers of "exporting water" in the form of crops such as alfalfa and almonds to Japan and China. Alfalfa is one of the state's biggest water users, far exceeding the water requirements of other crops grown in the state.

"We're not attacking California agriculture," said Adam Scow, California director for Food and Water Watch, an environmental group. "It's here to stay. But when we have less water and we have more droughts and we want to protect our estuary, certain practices are not going to make sense forever."

Connected crops

Coburn, however, says his thirsty alfalfa isn't some boutique crop - it's vital to San Joaquin Valley ranchers and dairies that put meat and milk in the refrigerators of millions of Californians. The drought has driven alfalfa prices to record highs, if it is available at all.

When Coburn told one farmer he wasn't going to grow any alfalfa this summer, "he literally was beside himself," Coburn said. "How am I going to feed my cows this summer?" the man asked him.

Farmers' point: Everything they grow requires water, and shorting them has consequences for everyone.

"We will see sizable impacts this year for a lot of those fruits and vegetables, sweet corn, melons, those things that you take for granted that are fresh," said Wenger, the farm bureau president.

"You like wine?" Coburn asked. "How many gallons of water do you think goes into a bottle of Two-Buck Chuck or a really nice varietal up north? Take your pick: 275 gallons."

Farmers have taken heat from environmentalists for going more heavily into permanent crops - trees and vines that produce grapes, almonds, pistachios, avocados and the like, and require steady watering over their decades-long life. They cannot be fallowed like melons or tomatoes for a year or two during droughts.

The public, however, seems to like tree crops. Last week, the Chipotle restaurant chain warned in its annual report that climate change could reduce avocado supplies and force it to stop serving guacamole. Such was the panic that the company had to clarify that a "guacapocalypse" was not at hand.

If climate-change models are borne out and California droughts intensify, the trade-offs are likely to be increasingly painful for consumers as well as farmers.

"When people say the farmers are using all the water," said Michael Dimock, head of Roots of Change, a San Francisco sustainable food group, "what they're really saying is the public is eating."